In a differentially pumped mass spectrometer system a sample and carrier gas are introduced to a mass analyser for analysis. One such example is given in FIG. 1, in which there exists a high vacuum chamber 10 immediately following a number of evacuated interface chambers, the actual number of such chambers depending on the type of system. In the example shown in FIG. 1, the system includes first, second and third evacuated interface chambers 12, 14 and 16.
The first interface chamber 12 is the highest-pressure chamber in the evacuated spectrometer system and may contain a gas inlet means through which ions are drawn from the ion source into the first interface chamber 12. The ion source may be at atmospheric pressure depending upon the ionisation method employed. The second interface chamber 14 and subsequent lower pressure chambers may contain ion optics and means of analysis known to those skilled in the art.
In this example, in use, the first interface chamber 12 is at a pressure of around 1-10 mbar, the second interface chamber 14 is at a pressure of around 10−3-10−2 mbar, the third interface chamber 16 is at a pressure of around 10−5-10−4 mbar, and the high vacuum chamber 10 is at a pressure of around 10−7-10−6 mbar.
To evacuate the chambers, in this example the low pressure chamber 10 is evacuated by a turbomolecular pump 20 exhausting to a backing pump 22 or another appropriate point on the vacuum system, the second and third interface chambers 14, 16 are evacuated by a compound vacuum pump 24 exhausting to the backing pump 22, and the first interface chamber 12 is evacuated by the backing pump 22. The backing pump 22 may be a relatively large, floor standing, rotary vane pump or other appropriate type of vacuum pump.
In this example, the compound vacuum pump 24 has two pumping sections in the form of two sets 30, 32 of turbomolecular stages, and a third pumping section in the form of a Holweck drag mechanism 34; an alternative form of drag mechanism, such as a Siegbahn or Gaede mechanism, could be used instead. Each set 30, 32 of turbomolecular stages comprises a number (four shown in FIG. 1, although any suitable number could be provided) of rotor and stator blade pairs of known angled construction. The Holweck mechanism 34 includes a number (two shown in FIG. 1, although any suitable number could be provided) of rotating cylinders, corresponding annular stators, and helical channels in a manner known per se.
A first compound pump inlet 36 is connected to the third interface chamber 16, and fluid pumped through the inlet 36 passes through both sets 30, 32 of turbo-molecular stages in sequence and the Holweck mechanism 34 and exits the pump via outlet 38. A second compound pump inlet 40 is connected to the second interface chamber 14, and fluid pumped through this inlet 40 passes through set 32 of turbo-molecular stages and the Holweck mechanism 34 and exits the pump via outlet 38. The compound pump 24 may include additional inlets, for example interstage the turbomolecular and Holweck pumping stages, if required to pump additional system chambers.
As fluid entering each compound pump inlet passes through a respective different number of stages before exiting from the compound pump, the compound pump 24 is able to provide the required vacuum levels in the chambers 14 and 16, with the backing pump 22 providing the required vacuum level in the chamber 12 and the turbomolecular pump 20 providing the required vacuum level in the chamber 10.
Utilising a compound pump to evacuate two or more adjacent chambers offers advantages in size, cost, and component rationalisation. However, in view of the conductance limitations of typical compound pumping arrangements performance is compromised in comparison to an arrangement where each of the intermediate chambers is evacuated using a bespoke vacuum pump directly mounted on to the respective intermediate chamber.
Depending on the type of mass spectrometer system, pumping performance can also be significantly affected when, as shown in FIG. 1, an additional gas load is introduced into one of the intermediate chambers 14 or 16 through, for example, a collision cell, gas reaction cell or ion trap. In the example shown in FIG. 1 the additional gas load is depicted as being introduced into chamber 16. To maintain pressures in this chamber a much higher level of pumping performance is now required at the chamber.
An aim of this invention is to provide a pumping arrangement for a plurality of chambers which offers the required level of performance without substantially increasing the size, cost or number of pumps in the pumping arrangement.